I think it's probaby "90% efficacy rate", which means actually:
>Say in a population on an isolated island there are 100 people
>And if they don't get vaccinated, then 10 will die
>But if 100% of them are vaccinated, then only 1 will die (ie 90% of the 10 people will survive)
That's efficacy.
But the goal ain't saving the vaccinated, no matter what they tell you. It's all about reducing the spread.
>Think of a virus as a kind of vampire infection
>you get the virus, you turn into a vampire & try to bite people & infect them
>but people fight back, that's the immune system, sometimes it wins against you
>this vampire disease is super strong, you win most fights, so do others infected, it spreads and spreads
>but a vaccine means you start doing worse
>so, akin to doping in a nuclear reactor, everything starts to slow down
>you successfully bite fewer before you yourself get well on your own
>same with the others
>Herd immunity is the idea that enough healthy people intentionally go vampire to provide the brakes on the spread
A disease fails once it can't get enough people infected successfully in time, before the infectee gets well (or dies).
